There have been several posts on this that have been quite helpful, but I think they were only looking at 1.2. I originally saw this post on Benjamin Day's blog. I tried it and it didn't work for me...then I realized the file names have changed. The steps are basically the same:

Copy the *.xsd files from src\NHibernate (there are three - nhibernate-configuration.xsd, nhibernate-generic.xsd, and nhibernate-mapping.xsd)

Copy them to \program files\Microsoft Visual Studio 9.0\Xml\Schemas

Rename them to include the -2.2 suffix (nhibernate-configuration-2.2.xsd, nhibernate-generic-2.2.xsd, and nhibernate-mapping-2.2.xsd)